In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. When a switch is designed to switch significant power, the transitional state of the switch as well as the ability to stand continuous operating currents must be considered. When a switch is in the on state its resistance is very low and very little power is dropped in the contacts. When a switch is in the off state its resistance is extremely high and even less power is dropped in the contacts. However, when the switch is actuated, the resistance must pass through a state where a considerable portion of the load's rated power is dropped in the switch, demanding robust switching devices. The electrical actuator for a motor, a magnet, a valve or lamp may therefore for example be activated by a power switch.
An objective for example in the automobile industry is to activate anything that for example rotates, slides, pumps or heats by means of power semiconductors. “Smart” power switching circuits, such as for example Freescale's “eXtreme switch” devices, may be used for example in vehicles, e.g. cars or trucks, for driving different types of loads such as for example bulb-lamps or DC-motors.
In a power switching apparatus, a power switching circuit may work together with a microcontroller unit or processing device which may control the power swtiching circuit by mean of some communication interface. Smart power switching circuits can be configured to perform additional functions, such as for example protecting lines against short-circuits, sensing the current flow at any given time and providing the sense current through terminal CSNS, load diagnostics, for example over-temperature detection or open-load detection, and load control, which may be adapted to requirements by means of pulse width modulation (PWM), and suppressing electromagnetic interference during the process of switching.
Especially current sensing of load current can be an important additional feature. In an environment where heavy loads as well as light loads may be switched, the smart power switch may be a low on-resistance power switch for the heavy loads and may especially be able to provide accurate current sense at light loads, such as light emitting diodes (LED). For example, a smart power switch in an automobile environment may have the ability to drive high-intensity discharge (HID) xenon and halogen lamps and light-emitting diode light sources with a single device, thus improving lighting efficiency, extending bulb life and reducing material costs.
As shown in FIG. 1, a smart power switching circuit 10 may contain a regular load or output current monitoring and over-current protection circuit. The current sense functionality is implemented by separating the power switch in a (main) power switching device 12, a mirror, sense device 14 and by usage of a differential or error amplifier 16 to form an accurate current sense with an accuracy of the current sense function only limited at low load currents and/or at low on-resistance of the power switch by an error value introduced by the error amplifier offset. Current sense feedback in this circuit is formed by the error amplifier 16 and transistor 18. Current sense feedback (negative feedback) may keep equal voltage potentials on the sources 20, 22 of main power switching 14 and sense 16 transistors. So current through drain-source path of the sense transitor 14 as well as through transitor 18 thanks to the error amplifier 16 is proportional to the load current (ILOAD). As shown, this current may be replicated by replica MOSFETs 24, 26, 28 to output with some current gain (M/N in this example).
When error introduced by the offset of the error amplifier is sufficient at load current of interest, an approach for improving current sense accuracy is for example two points calibration of the power switching circuit, which includes individually characterizing each power switching circuit and saving obtained coefficients for use in a program for a microcontroller which controls the power switching circuit and measures current sense output by an embedded analog-to-digital converter (ADC). Other common techniques for offset reduction are for example, just to name a few: analog/digital offset compensation, auto-zeroing, chopping, offset stabilization and their various combinations.